Method and apparatus for manufacturing ceramic devices

ABSTRACT

An apparatus for manufacturing a ceramic device from tape ( 21 ) in a green state comprises a cylindrical rod ( 23 ) as a first forming element, a first clamping system ( 241 ) for fixing a first end of the tape ( 21 ) with respect to the cylindrical rod ( 23 ) and a first mechanism ( 20, 231 ) for winding the tape ( 21 ) around the cylindrical rod ( 23 ) to preform the tape ( 21 ). The apparatus further comprises a second forming element ( 33 ), a second clamping system ( 351, 352 ) for fixing at least one end of the pre-formed tape ( 21 ) with respect to the second forming element ( 33 ), and a second mechanism ( 331, 351 ) which winds the pre-formed tape ( 21 ) around the second forming element ( 33 ).

This invention relates to a method and apparatus for manufacturingcurved elements of electro-active material. More particularly, itrelates to a method and apparatus for manufacturing electro-activeactuators curved around an axis which is itself curved.

Comparably large translation displacements have been recently achievedby using a curved, helical structure of coiled piezoelectric tape inwhich a multilayer tape is curved helically around an axis which isitself curved. Such devices are found to easily exhibit displacement inthe order of millimetres on an active length of the order ofcentimetres. These structures and variations thereof are described, forexample, in WO-01/47041 and in D. H. Pearce et al., Sensors andActuators A 100 (2002), 281-286.

These structures are ceramic devices of complex curved shape and arecurrently manufactured using inefficient low output methods which relyheavily on human labor. Other methods using extrusion processes aredescribed for example in WO-02/103819. However the described process maynot be feasible for all configurations of super-helical actuators.

Therefore it is seen as an object of the invention to provide a methodand apparatus for manufacturing twice coiled actuators with limitedhuman involvement and using flat ceramic tape as base material.

According to an aspect of the invention, there is provided a method ofand an apparatus for manufacturing ceramic devices including the stepsof transporting ceramic tape in a green state into the proximity of afirst forming element; fixing a first end of the tape with respect tothe first forming element; moving the tape and first forming elementrelatively to each other in a motion including simultaneously rotationaland translational movements, thus winding the tape around the firstforming element; removing the fixing of a first end thereby allowingseparation of the tape and first forming element to generate a helicallywound pre-formed tape; fixing at least one end of the pre-formed tapewith respect to a second forming element; and moving the pre-formed tapeand second forming element relatively to each other in a motionincluding at least a rotational movement, thus winding the pre-formedtape around the second forming element.

The invention preferably includes the step of and devices for moving thetape and the first forming element relatively to each other includingthe step of continuously pressing during the movement the tape onto thefirst forming element at a zone where the tape first contacts the firstforming element.

Furthermore the invention preferably includes the step of exerting aforce on the edge of the tape at a zone where the tape first contactsthe first forming element to prevent slippage of the tape relative tothe first forming element.

The invention preferably includes the step of and devices for closing asecond clamping element around the first end of the tape beforereleasing a first clamping element fixing the tape to the first formingelement.

Preferably, the clamping elements used for gripping the tape or otherdevices designed to exert pressure directly onto the tape arepneumatically operated or spring-loaded to simulate a “tactile” handlingof the tape.

The invention preferably includes the step of and devices for passingthe first forming element along an edge holding back the pre-formedtape.

Preferably the pre-formed tape is held at both ends using clampingelements with one of the clamping elements fixing one end of thepre-formed tape with respect to the second forming element while theother clamping element performs a rotational movement around the secondforming element.

The invention preferably includes the step of and devices for removingthe fixing with respect to the second forming element and transferringthe wound pre-formed tape onto a support structure; and placing thesupport structure into heated environment for drying and/or sintering.

The present invention is particularly advantageous used to manufacturetwice-coiled helices, particularly twice-coiled actuators ofpiezoelectric material.

These and other aspects of inventions will be apparent from thefollowing detailed description of non-limitative examples makingreference to the following drawings.

In the drawings:

FIG. 1 is a perspective view of a twice-coiled actuator as known;

FIG. 2 illustrates a first forming step in accordance with an example ofthe present invention;

FIG. 3A illustrates a step of releasing pre-formed tape from a firstforming element;

FIG. 3B illustrates a second forming step in accordance with an exampleof the present invention; and

FIG. 4 illustrates the transfer of formed tape onto a support structurefor drying.

FIG. 1 shows a known actuator 10 of the type described in WO-01/47041and Sensors and Actuators A 100 (2002), 281-286.

The actuator 10 has a curved portion 12 of multi-layer tape 11, forexample of a bimorph construction, that is wound helically around afirst axis 13 referred to as the minor axis. For illustration, the minoraxis is shown as a dashed line 13 in FIG. 1. The helically wound portionis further coiled into a secondary winding of about three quarters of acomplete turn. The axis 14 of this secondary winding is referred to asthe major axis and shown as a small dashed circle 14 with a centralsolid point again to facilitate description and illustration. The firstwinding is known as the primary winding or primary helix. The secondarywinding could be any curve and could exceed one turn and form a spiralor secondary helix. It is therefore usually referred to as secondarycurve. The tape is arranged on actuation to bend around the minor axis13. Due to the helical curve around the minor axis, such bending isconcomitant with twisting of the actuator 10 around the minor axis. Dueto the curve around the major axis 14, such twisting is concomitant withrelative displacement of the ends 111, 112 of the actuator 10. When usedas an actuator, one end 111 of the tape 11 may be fixed to a moveableobject (not shown) while the other end 112 of the tape 11 may be mountedonto a base plate 15.

The manufacture of the actuator 10 from a precursor ceramic tape is avery complex task usually performed by manual labor. It involves thesteps of slowly winding a helix of the tape around a cylindrical rodand, and after carefully removing the rod, placing the helix into a“sagger” that supports the helix in its coiled form during subsequentdrying until the ceramic tape is sufficiently stiff to support its ownweight. The coiled helix is then burned to remove binder and otherorganic components and sintered to form the ceramic actuator.

While the above steps can be performed manually, the process does notreadily lend itself for automation. The properties of the precursorceramic tape are such that, though pliable, it slackens readily and isunable to support its own weight.

Referring now to FIG. 2A, there is shown a conveyor system 20 thattransports a strip of precursor ceramic tape 21 to a cylindrical rod 23.The conveyor system 20 includes rollers 201 and support surfaces 202that guide the strip 21 close to the former 23. A smoothly contouredsurface 204 directs the tape 21 under the rod 23. When positioned at theconveyor system 20, an end block 231 supports the distal end of the rodto prevent it from bending and provides a rotary drive for rotating therod 23 and a linear drive for translating the rod 23. After a tap 211 ofsufficient length has passed a first pneumatically operated clamp 241closes and fixes the tape 21 against the rod 23. Then the rod 23 isrotated and translated as indicated, while more tape is pushed throughthe conveyor system 20. Thus the end block 23 and the conveyor system 20act as the a first mechanism for moving the tape 21 and the rod 23relatively to each other.

The rod 23 acts as a primary or first former. Its diameter is chosensuch that it matches the inner diameter of the primary helix.

The conveyor and the axis of the rod form an acute angle to feed tape atthis angle onto the rod. As the strip is pushed by the conveyor acrossthe rod 23, the rotation of the rod 23 causes the strip 21 to be woundaround it. Simultaneously with the rotation, the rod 23 is moved indirection of its axis. Both motions are synchronized such that the tapeis wound around the rod in a helical manner.

In FIG. 2B, the winding process of FIG. 2A is shown at a later stagewith the tape 21 being wound around the former 23 for approximately halfof its length. As the first clamp 241 is retracted from the conveyorsystem 20, a second spring-loaded clamp 242 is brought from below intocontact with the tape. The second clamp has a slightly concave surfacewith a stop pin 243. The concave surface maintains pressure on the tapewhile the stop pin 243 prevents the slippage of tape along the rotatingrod 23, thus ensuring that the helix remains tightly wound.

After the strip of tape 21 is fully wound around the primary former 23,the second end of the tape forms a second short stub (not shown) similarto the stub 211. The rod 23 is then moved by the linear drive of theblock 231 to a second forming element. During the transport both clamps241, 242 remain in place.

At the location of the second former 33, two clamps 351, 352 grip thetape at the stubs 211 as illustrated in FIG. 3A. The previous clamps241, 242 are released and the forming rod 23 is moved in axial directionin sliding contact with an edge 353. The first former 23 is shown as adashed outline. The first clamp 351 is fixed to the mounting block 331that carries the second former 33. It pushes the stub 211 at one end ofthe tape 21 against the outer edge of the inner cylinder 332 of thesecond former 33. The combined clamp 351 and second former 33 aresimultaneously rotated around the center axis of the cylindrical secondformer 33 and moved towards the second clamp 352 that rests immobileduring this movement. Thus, the first clamp 351 and the second clamp 352form the a second mechanism for moving said pre-formed tape and secondforming element relatively to each other. As the second former 33approaches the second clamp, more of the rod 23 slides over an edge 353and more windings are pushed from the distal end of the rod. Thewindings are wound around the inner cylinder 332 of the rotatingsecondary former 33 until the tape is completely stripped of the primaryformer. At this stage, which is illustrated in FIG. 313, the pre-formedtape 21 is wound completely around the inner cylinder 332 of secondformer 33 and held in place by the two clamps 351, 352.

The inner cylinder 332 is spring-mounted within a bore of an outercylinder 333 of the second former 33, such that pressure on the frontface of the inner cylinder causes it to retract into the bore while theadvancing sleeve of the outer cylinder pushes the wound tape 21 from theformer.

At this stage of the process the twice-coiled actuator of FIG. 1 iscompleted in its “green” state. However to stabilize the structure, thetape 21 has to be at least dried. To free the second former 33, thewound helix is transferred into a tray or array for further handlingoff-line.

In FIG. 4, the secondary former 33 is shown approaching a array 40 ofsaggers 41. As the secondary former meets a sagger 41, the front face ofits inner cylinder 332 (shown in FIG. 3 above) engages a matchingcylinder 431 within a recess area 432 of a sagger 41. As the secondaryformer moves closer to a sagger, the spring-loaded inner cylinder 332remains stationary and the outer ring 333 pushes the twice-coiled helixinto the recess 432 of the sagger. The clamps that grip the stubs orends of the tape open to release the helix. The outer boundary of therecess area 432 has a diameter that closely matches the nominal outerdiameter of the major helix of the actuator. The edges of the boundaryof the recess area are contoured or chamfered to assist the placing ofthe tape into the sagger. The sagger array 40 filled with fully formedgreen tape is then placed in a heated environment for drying andprepared for further processing stages such as sintering in an oven.

The various clamping devices described above are using pneumaticallyoperated actuators commercially available for example from Festo.Commercially available DC servo motors are used to generate othermovements of the components. All components are under the control of acomputer program stored in and executed from an Intel processor basedworkstation.

Variations of the above example are readily within the scope of askilled person. It is for example feasible to design the formingelements in a segmented manner to alter their diameter and hence thedimensions of the actuator. It is also possible to replace the secondforming element with a removable sagger and thus wind the pre-formedtape directly onto the sagger.

1. A method of manufacturing a ceramic device comprising: transportingceramic tape in a green state into the proximity of a first formingelement; fixing a first end of said tape with respect to said firstforming element; moving said tape and first forming element relativelyto each other in a motion including simultaneously rotational andtranslational movements, thus winding said tape around said firstforming element; removing said fixing of a first end thereby allowingseparation of said tape and first forming element to generate ahelically wound pre-formed tape; fixing at least one end of saidpre-formed tape with respect to a second forming element; and movingsaid pre-formed tape and second forming element relatively to each otherin a motion including at least a rotational movement, thus winding saidpre-formed tape around said second forming element.
 2. The method ofclaim 1, further comprising, during the step of moving said tape andfirst forming element relatively to each other, continuously pressingsaid tape onto said first forming element at a zone where said tapefirst contacts said first forming element.
 3. The method of claim 2,further comprising, during the step of moving said tape and firstforming element relatively to each other, exerting a force on the edgeof the tape at a zone where said tape first contacts the first formingelement to prevent slippage of said tape relative to said first formingelement.
 4. The method of claim 1, further comprising, between the stepof moving said tape and first forming element relatively to each otherand the step of removing said fixing of a first end, closing a secondclamping element around the first end of the tape.
 5. The method ofclaim 1, wherein the step of separating said tape and first formingelement comprises moving the first forming element relatively to an edgeholding back the pre-formed tape.
 6. The method of claim 1, wherein,during the step of moving said pre-formed tape and second formingelement relatively to each other, the pre-formed tape is held at bothends using clamping elements, one of said clamping elements fixing oneend of said pre-formed tape with respect to the second forming element,the other clamping element performing a relative rotational movementaround said second forming element.
 7. The method of claim 1, furthercomprising the steps of removing the fixing with respect to the secondforming element and transferring the wound pre-formed tape onto asupport structure; and placing said support structure into environmentfor drying.
 8. The method of claim 1, wherein the tape is handled bypneumatically operated devices.
 9. The method of claim 1, wherein thestep of moving said tape and first forming element relatively to eachother comprises rotating the first forming element and translating theunwound portion of the tape towards the first forming element.
 10. Themethod of claim 9, wherein the step of moving said tape and firstforming element relatively to each other further comprises translatingthe first forming element along an axis about which the tape is wound.11. The method of claim 1, wherein, in the step of moving said tape andfirst forming element relatively to each other, the tape is woundhelically around the first forming element.
 12. The method of claim 1,wherein the first forming element is cylindrical.
 13. The method ofclaim 1, wherein the tape comprises layers of electro-active material.14. An apparatus for manufacturing a ceramic device from tape in a greenstate, the apparatus comprising a first forming element; a firstclamping system for fixing a first end of said tape with respect to saidfirst forming element; a first mechanism for moving said tape and firstforming element relatively to each other in a motion includingsimultaneously rotational and translational movements, thus winding saidtape around said first forming element to form a preformed tape; asecond forming element; a second clamping system for fixing at least oneend of said pre-formed tape with respect to said second forming element;and a second mechanism for moving said pre-formed tape and secondforming element relatively to each other in a motion including at leasta rotational movement, thus winding said pre-formed tape around saidsecond forming element.
 15. The apparatus of claim 14, wherein the firstclamping system includes a spring- loaded surface that in operationcontinuously presses the tape onto said first forming element at a zonewhere said tape first contacts said first forming element.
 16. Theapparatus of claim 15, wherein the spring-loaded surface is combinedwith a force-transmitting member adapted to contact the edge of the tapeat a zone where said tape first contacts the first forming element toprevent slippage of said tape relative to said first forming element.17. The apparatus of claim 14, further comprising a stripping edgemaking contact to the first forming element to separate said firstforming element and the pre-formed tape.
 18. The apparatus of claim 14,wherein the second clamping system includes a first clamping element forfixing a first end of the tape to the second forming element and asecond clamping element for fixing the other end of the tape, the secondmechanism being arranged to relatively rotate the second clampingelement around said second forming element.
 19. The apparatus of claim18, further comprising one or more saggers having recesses adapted tosupport the formed tape during drying.
 20. The apparatus of claim 14,wherein the clamping devices are pneumatically operated.
 21. Theapparatus of claim 14, wherein the first forming element is acylindrical rod.
 22. The apparatus of claim 14, wherein the firstmechanism comprises a rotary drive arranged to rotate the first formingmember and a conveyor system arranged to translate the tape towards thefirst forming member.
 23. The apparatus of claim 22, wherein the firstmechanism further comprises a linear drive arranged to translate thefirst forming member along an axis about which the tape is wound.
 24. Aceramic structure made using methods or apparatus in accordance withclaim
 1. 25. The ceramic structure of claim 24 being an electro-activeceramic structure.